1. Field of the Invention
The present invention relates to an exposure apparatus used to manufacture a device such as a semiconductor device.
2. Description of the Related Art
An exposure apparatus which exposes a substrate to radiant energy, such as light, via the pattern of a mask or reticle (to be generically referred to as a “reticle” hereinafter) and a projection optical system is used in lithographic process to manufacture, for example, a semiconductor device, liquid crystal display device, or thin-film magnetic head.
Along with the micropatterning and an increase in the density of integrated circuits, the exposure apparatus is required to project the circuit pattern on the reticle surface onto the substrate surface by exposure with high resolving power. The projection resolving power of the circuit pattern depends on the exposure wavelength and the numerical aperture (NA) of the projection optical system. In view of this, various efforts are made by increasing the NA of the projection optical system, changing the wavelength of the illumination light from the g-line to the i-line and from the i-line to the excimer laser oscillation wavelength, and shortening the excimer laser oscillation wavelength to 248 nm, 193 nm, and even 157 nm.
At the same time, the exposure area is increasing. A stepper and scanner are available as means for achieving this aim. The stepper reduces and projects roughly square shot regions onto a substrate by full plate exposure. The scanner accurately exposes relatively large rectangular or arcuated slit shot regions by relatively scanning the reticle and the substrate at high speed. Since the scanner aligns the surface position of the substrate for each slit region to be exposed to light with the image plane, it can reduce the influence of the substrate flatness. The scanner can also increase the shot region area and the NA using a lens equivalent to that of the stepper. Hence, the scanner is becoming the mainstream of the exposure apparatus.
A scanning exposure apparatus commonly called a scanner aligns, in real time, the substrate surface with the image plane in a slit to be scanned and exposed to light. For this purpose, the scanner measures the substrate surface position before the exposure slit using a gap sensor such as an oblique-incident-light surface position measuring device, air microsensor, or capacitance sensor, and moves the substrate. The exposure slit has a plurality of measurement points especially in the longitudinal direction (a direction perpendicular to the scanning direction) to measure not only the level but also the tilt of the surface.
Referring to FIG. 9, three measurement points of the surface position measuring device are provided before and after a scanning exposure slit. Referring to FIG. 10, five measurement points are provided. Providing measurement points before and after the slit allows substrate surface position measurement before exposure irrespective of whether the exposure scanning direction is positive or negative.
There is proposed a method of attaining high focus correction accuracy with respect to the depth of focus in reduction, thus improving the yield per substrate. This method calculates the substrate surface position in advance using a surface position measuring device arranged separately from the exposure apparatus, and drives the substrate under focus/tilt control using the calculated surface position.
When rectangular shot regions are sequentially transferred onto a circular substrate, some shot regions may partially fall outside the substrate in its periphery, as shown in FIG. 5. As shown in FIG. 6, even in a so-called multichip arrangement in which a plurality of chips constitute a shot region, some chips may fall outside the substrate in its periphery. This makes it necessary to expose a shot region 501 (non-rectangular portion), which partially falls outside the substrate, to light by the usual method.
When one chip constitutes a shot region as in a CPU, a photoresist remaining on the periphery of the substrate is removed upon clamping the substrate surface by, for example, ion implantation and RIE. The removed photoresist may transfer to the pattern of a chip, causing a pattern defect or dimensional error. This degrades the chip yield, so non-rectangular portions are also exposed to light to remove any unnecessary resist.
There is a method of determining, in advance, the validity of a measurement point where the substrate surface position is measured in scanning measurement from layout information of, for example, a substrate and chip, and measuring and correcting the focus while dynamically switching a surface position measuring sensor during scanning exposure. This method attains accurate surface position measurement and favorable exposure in a non-rectangular shot region which partially falls outside the substrate in its periphery. Japanese Patent Laid-Open No. 10-116877 discloses this method.
However, along with the recent spread of the network society, a stricter demand has arisen for LSIs with high performances (e.g., an increase in the degree of integration, reduction in chip size, high speed, and low power consumption). To meet this demand, more micropatterned and multilayered interconnections have been developed for each generation in accordance with the International Technology Roadmap for Semiconductors (ITRS). This poses a new problem associated with the accuracy of aligning the substrate surface to be exposed to light with a best image plane, because the depth of focus extremely decreases along with the trend toward micropatterning. The surface position measurement accuracy in a shot region suffers particularly when the substrate has a large variance of the surface shape (low flatness).
The control requirement for the substrate flatness is generally 1/10 to ⅕ the depth of focus of an exposure apparatus. If the depth of focus is 0.4 μm, an in-plane variation occurs on the order of 0.04 μm to 0.08 μm. As shown in FIG. 11, assume that the substrate is driven based on information on measurement points FP1 to FP3 arranged at a predetermined interval. Since information on the substrate surface position is absent between the measurement points, it is defocused by an amount of shift Δ from a plane calculated from the measurement points FP1 to FP3. This problem is also attributed to a so-called focus sampling error.
To solve this problem, the influence of a focus accuracy variation may be reduced by minimizing the focus sampling interval and forming a multi-point surface position measuring device which strictly controls the positions of measurement points. However, the mechanical tolerance/adjustment tolerance increases in proportion to an increase in the number of surface position measurement points. This raises the degree of manufacturing difficulty, resulting in an increase in cost in a broad sense. The mechanical tolerance/adjustment tolerance of a multi-point surface position measurement position on the substrate is directly translated into an individual variation for each apparatus. For example, in an underlying pattern with a large difference in reflectance as represented by a copper interconnection, a small difference between measurement positions varies the focus correction accuracy.